Absolute rate constants for the reactions of substituted methylperoxyl radicals with ascorbate, urate, trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), and TMPD (N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine) have been determined by pulse radiolysis is different solvents. In water-alcohol or water-dioxane solutions, the rate constants for trihalomethylperoxyl radicals generally increase with increasing water content. The rate constant for reaction of CCl{sub 3}O{sub 2}{center dot} radicals with trolox was measured in water, MeOH, i-PrOH, t-BuOH, ethylene glycol, diethyl ether, dioxane, acetone, acetonitrile, formamide, dimethylformamide, pyridine, and CCl{sub 4}. The rate constants were found to correlate well with a two-parameter equation that includes the dielectric constant of the solvent and the coordinate covalency parameter, a measure of the proton-transfer basicity of the solvent. Kinetic isotope effects in H{sub 2}O/D{sub 2}O of about 2 and activation entropies of about -1o eu for reduction of RO{sub 2}{center dot} by the organic reductants indicate that electron transfer to the peroxyl radical is concerted with the transfer of proton from the solvent to the incipient hydroperoxide anion.